fix(gatsby): Fix Loki query operators special casing

[stefanprobst]

There are subtle differences between Loki and Sift query operators that we are trying to account for. This fixes some of those:

• Sift allows querying arrays with $eq and $ne, which translates to $contains and $containsNone in Loki
• The special case for $nin added false, which should be undefined. This also needed removing cloning the query args with JSON.parse/stringify, because undefined=>null, but it does not seem to serve a purpose anymore anyway.
• The fix for { $ne: true } now handles nesting levels > 2, e.g. { "foo.bar.baz": { $ne: true } }
• The conversion to dotted query fields was producing { "": {} } for an empty object.

[freiksenet]

CC @Moocar

[Moocar]

@stefanprobst thanks so much! This looks great. The only thing I'm unsure of is the removal of JSON.parse/stringify as that was there before my time. But seeing as its removal doesn't break any tests, I think you're right in removing it.

[Moocar]

@freiksenet looks like I don't have merging rights, so I'll leave it up to you to merge (I approve).